Aspartate is an important excitatory transmitter in the brain, as well as a protein component and metabolic intermediate. An aspartate binding method was developed for use in brain which allows study of aspartate receptors. Two binding sites were demonstrated, differing in their sodium dependence, Kd, inhibition by other amino acids, and in their regional distribution. The ability to measure neuronal binding sites (receptors) opens up the possibility of studying interactions with other neurotransmitters, cotransmitters and neuromodulators, as well as the possibility of exploring aspartate pathways in areas of brain where detection of changes in aspartate content would not be possible.